1 Year of Unlimited Service FREE + Taxes When Switching from Another US Carrier(repost)
This deal may expire within the next few days, so we are re-posting it.
To get this deal, you must switch your service from a major US carrier to Sprint, and

You are not in a service contract with your existing mobile service provider, and

Your phone(s) are fully paid off and not on lease, and

Your credit is good, and

You get the deal directly from Sprint as described below (not in-store).

Confirm your account has no active contracts (you can cancel any time).

Call 866-782-8777 to sign up with Sprint.
Mention the Free Unlimited for a Year promotion.
Ask them to create a new Sprint account for you, and run any necessary credit checks.
If you are keeping your existing phone(s), skip to step 4.

If you decide to buy a new phone instead of bringing your existing phone to Sprint, make sure it's a full price buy (no lease and not connected to any Sprint plan).
Make sure the phone is "unlocked" and has never been active on Sprint.
For example, if you buy a new iPhone from an Apple store, have them use your old Verizon, T-Mobile, or AT&T SIM card to activate the phone (do not use a Sprint SIM card to activate the phone).
The Apple Store can give you a Sprint SIM card to take with you, but do not put the SIM card in the phone.

Get Sprint SIM cards for your phone(s).
This can be done by doing 1 of these:

When talking to the Sprint sales rep, they can mail cards to you for $10.

At an Apple store:
Many Apple stores will give Sprint SIM cards to you at no cost, just by asking.

At a Sprint store:
Ask for Sprint SIM cards from your local Sprint store.
They may charge a fee.
Be sure they do not activate the SIM card, and it comes still attached to its packaging.

Transfer your existing phone number(s) and phone(s) to Sprint by calling 866-782-8777.
This will activate the plan.
Remember, each phone number must be transferred from another major wireless provider in the US (AT&T, Verizon, T-Mobile, etc).
You cannot sign up new phone numbers on your free plan.

Mark your calendar for January, 2019.
Sprint will switch your plan to full price ($60/mo for the first line, $40/mo for the second, and $30/mo each additional line).
Re-negotiate or change plans then.

For the next year, your actual monthly bill will be free plus government taxes, which can come to about $3 or $5 per month per line.
Two lines, for example, would cost about $6 to $10 per month after taxes & fees, far less than Verizon's price of $130/mo plus taxes for 2 lines.
This deal may expire soon, so consider activating the service as soon as you can (before December 31).
After 4 months, you can purchase or lease a new phone and remain on the 1-year free plan.
Call Sprint customer service to confirm this.
When your free Sprint service is active, do not:

Within the first 4 months, do not cancel any phones on your account.
If you switch phones or cancel service within 4 months, you will lose the remainder of your 1-year free service.

Do not apply any upgrades to your account, as it may ruin your free status for the year.

Do not buy phones from a Sprint store which could mess up your free plan.

Optional phone insurance can be purchased through Sprint customer service or AppleCare.
For reference, here is a copy of the promotion that will arrive if you order SIM cards directly from Sprint.